I think so. If the protfolio is a custom post type (which I guess it is).
You can select it on the content options panel. Here is it in the documentation: http://wp-relatedpostspro.demo.wp-dreams.com/docs/#content_options
Turn off the “Return posts” and “Return pages” options and drag the portfolio (or whatever it's called) custom post type from the right column to the left.Best,
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